Laser line projectors find extensive use in machine vision applications. In these applications, the line of light is projected on a surface and light reflected from the surface is received by a detector. The surface is scanned relative to the detector and the detector output is electronically processed to build up a three-dimensional (3D) image of the surface. It is very important in such an application that light uniformity in the length direction of the projected line be uniform. The more uniform the illumination on the surface, the more faithful a reproduction of the surface the 3D-image will be.
The most common laser line projectors used in machine vision applications are based on a lens having an acylindrical surface (acylindrical lens) usually referred to by practitioners of the art as a Powell lens, after the inventor. Such a lens and an arrangement for using the lens for projecting a line of light are described in detail in U.S. Pat. No. 4,826,299, the complete disclosure of which is hereby incorporated herein by reference. Later variations and applications of the Powell lens are described in U.S. Pat. Nos. 5,283,694; 5,629,808; 7,167,322; and 7,400,457, among others.
A typical basic configuration of a laser line projector includes a diode-laser delivering a laser-beam characterized as having a fast-axis and a slow-axis perpendicular to each other. The diode-laser is followed by a positive lens, and then the acylindrical beam-shaping lens or Powell lens.
The Powell lens itself is characterized in having a first axis in which acylindrical surface has optical power, and a second axis, perpendicular to the first axis, in which the acylindrical surface has zero optical power. The diode-laser is invariably arranged such that the fast- and slow-axes of the diode-laser are aligned precisely (at zero degrees) with the respectively first and second axes of the Powell lens, or vice-versa. The Powell lens spreads the laser beam in the first-axis of the lens such that the power in the beam is spread linearly as a function of spread-angle (fan angle) to provide a uniform or “flat-top” illumination along the spread beam. The positive lens typically configured and positioned to create a focus in the second axis to provide a uniform line of light at about the focus position, i.e., within the focal depth. The positive lens can also be positioned to, collimate or diverge, the beam in the other axis.
In this line projection arrangement the Powell lens is designed for a particular laser-beam size incident on the lens. If the beam incident on the lens does not match this size, the lens will not provide optimum uniformity along the line of light. Indeed, the uniformity of illumination in the line is sensitive even to relatively small variations in beam size incident on the lens.
By way of example FIG. 1 schematically illustrates calculated intensity as a function of fan-angle and beam size in a line spread by a Powell lens optimized for a beam size of 2 mm. The intensity for a beam of the nominal (optimum) beam size is depicted by a fine solid line. The intensity distribution for a beam size 15% smaller than nominal is depicted by a bold dashed line. The intensity distribution for a beam size 15% larger than nominal is depicted by a bold solid line. This distribution is usually termed a “bat-ear” distribution or a “pitchfork” distribution.
A manufacturer of laser line projectors is required to provide projected lines of diverse wavelengths and powers to satisfy the demand of different users. In order to satisfy such diversity, a manufacturer must employ different diode-lasers possibly from different manufacturers in a line-projector product line. This will result in a fixed optical arrangement of the type described above with a range of different beam-sizes at the Powell lens. Absent measures to deal with this, a wide, unacceptable variation in intensity distribution would result.
Various solutions to the problem of varying beam characteristics of diode-lasers are employed. By way of example the beam size may be manipulated by additional optical elements between the diode-laser and the Powell lens, i.e., a zoom lens may be used as the positive lens. Diode-lasers of any one type may be sorted to find those having beam-divergence divergence within a design tolerance. In addition, Powell lenses are often reconfigured (re-polished), by trial and error, to match particular diode-laser beam characteristics. These solutions, however effective, can consume a large amount of time or be costly implement. There is a need for a simpler solution for accommodating a wide range of diode-lasers in a particular diode-laser line projector design.